Fluorescent organic molecules typically suffer from disadvantages that include photo-bleaching, different excitation irradiation frequencies, and broad emissions. However, the substitution of fluorescent organic molecules with quantum dot semiconductor nanoparticles may circumvent these limitations.
The size of a semiconductor nanoparticle dictates the electronic properties of the material, with the band gap energy being inversely proportional to the size of the semiconductor nanoparticles as a consequence of quantum confinement effects. Different-sized quantum dots may be excited by irradiation with a single wavelength of light to give a discrete fluorescence emission of narrow band width. Further, the large surface-area-to-volume ratio of the nanoparticle typically has a profound impact upon the physical and chemical properties of the quantum dot.
Nanoparticles that include a single semiconductor material usually have modest physical/chemical stability and consequently relatively low fluorescence quantum efficiencies. These low quantum efficiencies arise from non-radiative electron-hole recombinations that occur at defects and dangling bonds at the surface of the nanoparticle.
Core-shell nanoparticles comprise a semiconductor core with a shell material of typically wider band-gap and similar lattice dimensions grown epitaxially on the surface of the core. The shell reduces defects and dangling bonds from the surface of the core, that confines charge carriers within the core and away from surface states that may function as centers for non-radiative recombination. More recently, the architecture of semiconductor nanoparticles has been further developed to include core/multishell nanoparticles in which the core semiconductor material is provided with two or more shell layers to further enhance the physical, chemical and/or optical properties of the nanoparticles.
The surfaces of core and core/(multi)shell semiconductor nanoparticles often possess highly reactive dangling bonds that may be passivated by coordination of a suitable ligand, such as an organic ligand compound. The ligand compound is typically either dissolved in an inert solvent or employed as the solvent in the nanoparticle core growth and/or shelling procedures that are used to synthesise the quantum dots. Either way, the ligand compound chelates the surface of the quantum dot by donating lone pair electrons to the surface metal atoms, which tends to inhibit aggregation of the particles, protect the particle from its surrounding chemical environment, provide electronic stabilisation, and may impart solubility in relatively non-polar media.
The widespread application of quantum dot nanoparticles in aqueous environments (i.e., media comprised primarily of water) has been restricted by the incompatibility of quantum dots with aqueous media, that is, the inability to form stable systems with quantum dots dispersed or dissolved in aqueous media. Consequently, a series of surface modification procedures have been developed to render quantum dots aqueous compatible, i.e., dots that may disperse homogeneously in water or media comprised primarily of water.
A procedure widely used to modify the surface of a quantum dot is known as “ligand exchange.” Lipophilic ligand molecules that inadvertently coordinate to the surface of the quantum dot during core synthesis and/or shelling procedures are subsequently exchanged with a polar/charged ligand compound of choice.
An alternative surface modification strategy interchelates polar/charged molecules or polymer molecules with the ligand molecules that are already coordinated to the surface of the quantum dot.
Current ligand exchange and interchelation procedures may render the quantum dot nanoparticles compatible with aqueous media but typically result in materials of lower quantum yield and/or substantially larger size than the corresponding unmodified quantum dot.